Cyclosporine (CsA) is a potent immunosuppressive agent which has been utilized to prevent solid organ transplant rejection and Graft versus Host disease following bone marrow transplantation. The precise subcellular mechanism of action of CsA is unclear at present. Recent studies from our laboratory have demonstrated that CsA binds to and inhibits calmodulin, a 17 kD calcium- dependent protein involved in many cellular activities. These data, coupled with increasing evidence of the role of calcium- dependent processes in T lymphocyte function, have led us to hypothesize that CsA inhibition of calmodulin and/or other calcium-dependent proteins may largely explain its immunosuppressive effects of T lymphocytes and possibly macrophages. The studies outlined in this research proposal will directly define the interaction of CsA and its active and inactive derivatives with the calcium-dependent proteins (proteins kinase C, calmodulin, phospholipase A2 and phospholipase C) and their acceptor proteins such as cyclic nucleotide phosphodiesterase. These studies will also define the immunosuppressive effects of a number of classes of calcium-calmodulin antagonists primarily on T lymphocyte subset generation (T helper cytotoxic and suppressor cells). These agents will be analyzed both alone and in combination with CsA. These studies will identify the specific calcium-dependent proteins which interact with CsA and their relative important in the CsA effect on T lymphocytes. These studies will also offer an indication of the relative importance of calcium-dependent processes in general and the calcium-dependent proteins in particular for the function and generation of the various T lymphocyte subsets in the immune response.